I haven't done my frame, yet, but will probably get to it in the Spring. My frame is older so I'll use some mechanical means to remove most of the rust, initially. After that, I'll treat the steel and rust with diluted Phosphoric acid. The cheapest source I can find for the acid is dehumidifier cleaner or water softener cleaner. I apply the acid with a handheld spray bottle. Give it some time to react with the rust, respray with the mixture and rinse. Any rust spots will be white when dried. On a trailer frame, I'll put on a coat of primer and then topcoat with a good enamel.
If I parked on the beach or lived close to the Coast, I'd probably use an industrial primer specific to a paint system and topcoat with a urethane or other polymer like POR-15.